Microstructure Evolution And Strengthening And Toughening Mechanism Of Tungsten By High Pressure Torsion

Refractory metal tungsten has wide applications in many fields such as aerospace,national defense,military,and nuclear industry due to its excellent comprehensive mechanical properties.As the demand for high-performance materials in the new era is increasing,existing materials cannot meet the performance requirements under extreme conditions.At present,industrial pure tungsten often undergoes secondary processing such as rolling and extrusion,but there are still problems such as high ductile-brittle transition temperature,coarse grains,and low density.There is a large disparity between the frontier materials with industrial pure tungsten and limitations on the application and development in many fields.The high pressure torsion（HPT）process can produce severe shear deformation and densify the material effectively,leading to a fine-grained structure with a non-equilibrium grain boundary and having a significant effect on improving the overall performance of pure tungsten materials.HPT process is used to prepare an ultrafine-grain material with excellent comprehensive performance,which can broaden the application field of refractory metal tungsten and promote the engineering application of high-performance materials.In this paper,a half-restricted HPT die was used to test of pure tungsten with different turns at a blank temperature of 550°C and mold temperature of 350°C.Metallographic observation,X-ray diffraction analysis,EBSD and TEM techniques were used to study The influence of different turns on the microstructure morphology,deformation substructure,grain orientation and grain boundary evolution of pure tungsten materials.The results show that with the increase of effective strain,the microstructure is gradually refined and the grain is elongated in the direction of torsion.The dislocation density within the structure increases significantly from 1.18×10¹⁴m^-2in the initial sample to 3.89×10¹⁴m^-2 in 5 turns sample.When the strain increases to a certain extent,the grain size and orientation in the microstructure tends to be evenly distributed,along with dynamic recrystallization.The transition from low-angle grain boundaries to high-angle grain boundaries can be observed clearly during HPT processing.The proportion of high-angle grain boundaries（>15°）after 5 turns HPT deformation increased to 62.1%.In the early stage of HPT,the deformation was dominated by intra-grain dislocation slip.When the grains are refined to near the mean free path of the dislocations,the deformation is dominated by grain boundary sliding,with the formation of a large number of non-equilibrium grain boundaries.The microhardness,nanoindentation test and isochronal annealing test of pure tungsten by HPT are used to analyze the low-temperature toughening mechanism and recrystallization thermal stability,which combined with the evolution characteristics of the microstructure of the material.The results show that the microhardness and deformation resistance of pure tungsten are improved with the increase of equivalent strain.As a result,the material has high strength and toughness.When the torsion number increases to 5 turns,the hardness values at the center and edge reach598.6±5HV and 662.2±6HV respectively,increasing by 39.1%and 54.0%compared to the original sample.The material strength and uniformity are enhanced.With the increase of torsion turns,the modulus of elasticity decreases,which indicates that the low-temperature brittleness of tungsten is improved.In addition to the classical dislocation strengthening and fine-grain strengthening,the strengthening model of HPT deformation of pure tungsten needs to consider the effect of non-equilibrium grain boundaries,and strengthening mechanisms are still dominated by dislocation strengthening.Annealing tests reveals that the recrystallization temperature is still at a high value about 1325°C¹350°C in HPT samples,with the average grain size about4¹0μm.The grain size consistency of the high-effective strain specimens after annealing is better in the radial direction,and the thermal stability of pure tungsten is relatively improved.